Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Chemistry pH of solution.

  1. Apr 12, 2010 #1
    Here is a problem I was working on but I do not have a solutions manual because Its an old text book. I would just like someone to look over my work.


    1. The problem statement, all variables and given/known data
    Calculate the pH of a 0.67 M NaClO solution.


    NaClO -> Na+ + ClO-

    ClO- + H2O -> HCLO + OH-

    Kw = 1.0*10^-14
    Ka HCLO = 3.5*10^-8

    Could you please check if these constants are correct.

    2. Relevant equations

    Kb = Kw/Ka HCLO

    pH = -log [H+]


    3. The attempt at a solution

    Kb = Kw/Ka HCLO = 1.0*10^-14/3.5*10^-8 = 2.86 * 10^-7

    With Kb we can solve concentrations for OH-

    Kb = [HClO] [OH-] / [ ClO-]
    2.86 *10^-7 = (x)(x) / (0.67-x) Quadratic Formula
    x = 4.38 * 10 -4 M
    [OH-] = 4.38 * 10^-4 M

    [H+] [ OH-] = 1.0 *10^-14
    [H+] = 1.0 *10^-14 / 4.38 * 10^-4 M
    = 2.28 * 10^-11 M

    pH = - log[H+]
    = - log( 2.28 * 10^-11)
    = 10.64


    This is a reasonalbe result.

    My last question is how do I turn off overtype when I am preparing a new thread?
    Thank you for your time.
     
  2. jcsd
  3. Apr 12, 2010 #2

    Borek

    User Avatar

    Staff: Mentor

    Looks OK. My sources give pKa for hypochlorous acid as 7.53.

    No idea what you mean.

    --
    methods
     
  4. Apr 12, 2010 #3
    So if the pKa is 7.53 should I use a different Ka?
     
  5. Apr 12, 2010 #4

    Borek

    User Avatar

    Staff: Mentor

    Not necesarilly. These values quite often differ between sources. If you are given pKa value, use it in calculations. If you ask for help and ask others to check your results - list pKa value that you used.

    --
    methods
     
  6. Apr 12, 2010 #5
    I agree thank you
     
  7. Oct 27, 2010 #6
    I think what she means by overtype is, when typing out a post, if you need to go back and make a correction to what you typed your correction eats away your typed text.

    For your information, I had the same problem and came looking here for an answer, then I realized that my problem had been fixed. After some cursory exploration, I found that there is an "Insert" key on my keyboard right next to my left arrow key, that turns on overtype. Hit it again to turn it off.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook